Abstract The tetraspanins comprise a class of four-pass transmembrane proteins that have acquired a variety of discrete, yet poorly understood, functions in mammalian physiology, playing essential roles in the immune, nervous, cardiovascular, reproductive, and auditory systems, as well as in infectious disease processes, tumor metastasis, and the development of organ systems. The functions of tetraspanins are mediated through their direct interaction with partner proteins, and one of the most critical functions of a tetraspanin is the regulation of B cell signaling via the interaction between the tetraspanin cluster of differentiation 81 (CD81) and the B cell co-receptor, cluster of differentiation 19 (CD19). Altered expression and signaling of CD19 causes defects in B cell development and has been implicated in the development of immunodeficiency, B cell malignancies, and autoimmunity. However, the mechanism by which CD81 regulates the trafficking and signaling of CD19 is unclear, largely due to a lack of structural and biochemical data. Therefore, I propose to investigate the molecular mechanism of tetraspanin regulation of CD19 through: i) structural characterization of the CD19- CD81 complex, and ii) functional studies of CD19 trafficking to the cell surface and signaling after B cell activation in cell-based assays. A better understanding of the molecular mechanism underlying CD19-CD81 function will have broad biomedical implications, offering novel insights into B cell biology and signal transduction mechanisms, tetraspanin function, and how to better target the B cell signaling pathway for novel therapeutics. Under the F31 fellowship award, I will receive exceptional training in structural biology, membrane protein biochemistry, and assay development under the guidance of Dr. Kruse and Dr. Blacklow at Harvard Medical School. My technical training will be accompanied by training in experimental design, mentorship, public speaking, biomedical ethics, scientific writing, and opportunities to present my research at scientific meetings. This training plan is designed to lead to a successful and productive graduate education and will provide an outstanding foundation for achieving my long-term goal of becoming an independent researcher focused on transmembrane signaling.